Before we discuss how we determine the critical engine on a multi-engine airplane, it is important to clearly define the term critical engine.
The critical engine is the engine which, when lost, most adversely affects the controllability of the airplane. This definition focuses specifically on aircraft control, not performance, climb capability, or altitude.
To determine which engine is the critical engine, we must analyze the aerodynamic effects that act on a twin-engine airplane during asymmetric thrust conditions. These effects explain why the loss of one engine results in a more severe control challenge than the loss of the other.
There are four primary aerodynamic factors used to determine the critical engine: P-Factor, Accelerated Slipstream, Spiraling Slipstream, and Torque.
An easy way to remember these factors is the acronym PAST.
Each of these aerodynamic factors contributes to the yawing and rolling moments that occur when an engine fails. Together, they illustrate which engine creates the most adverse control condition when it becomes inoperative, and why that engine is identified as the critical engine.
NOTE: The explanations and diagrams associated with the PAST factors assume a conventional twin-engine airplane, meaning both propellers rotate in the same direction—typically clockwise when viewed from the cockpit.
When an engine is lost, the airplane immediately tends to yaw and roll toward the inoperative engine due to asymmetric thrust. In a conventional twin-engine configuration, the loss of the left engine produces the most pronounced yawing and rolling tendency. The aerodynamic factors described below explain how this condition is amplified and why the left engine is considered the critical engine.
P-Factor (yaw)
The yawing effect of P-Factor becomes most noticeable at high pitch attitudes, such as during takeoff, initial climb, or slow flight at high power settings. In these conditions, the propeller disc is no longer perpendicular to the relative airflow, which causes an imbalance in thrust production across the propeller blades.
P-Factor occurs because the descending blade of the propeller operates at a higher angle of attack than the ascending blade. As a result, the descending blade produces more thrust. This creates an asymmetric thrust force that acts sideways relative to the aircraft’s longitudinal axis, producing a yawing moment.
In a conventional twin-engine airplane, where both propellers rotate clockwise (as viewed from the cockpit), the descending blade of each propeller is located on the right side of the propeller disc. This means that the thrust line of each engine is shifted slightly to the right of the engine’s centerline.
When both engines are operating normally, these yawing forces are balanced. However, when one engine fails, the remaining engine’s P-Factor becomes critically important in determining controllability.
In the case of left engine failure, the operating right engine produces thrust that acts farther from the aircraft’s centerline due to P-Factor. This results in a larger yawing moment toward the inoperative engine, requiring greater rudder input to maintain directional control. This condition significantly increases the workload on the pilot and directly contributes to the identification of the critical engine.
By contrast, when the right engine fails, the remaining left engine’s P-Factor produces a yawing moment that is closer to the aircraft’s centerline. The resulting yaw is less severe and requires less corrective rudder input.
For this reason, P-Factor is one of the primary aerodynamic reasons why the left engine is considered the critical engine in a conventional twin. Its influence is most pronounced at low airspeeds, high power settings, and high angles of attack—exactly the conditions where directional control margins are smallest.
Understanding the role of P-Factor is essential for recognizing why the critical engine presents the greatest controllability challenge during engine-out scenarios, particularly during takeoff and climb.
Accelerated Slipstream and the Critical Engine (Roll)
The rolling effect of accelerated slipstream is created by the descending blade of the propeller, which pushes a stream of accelerated airflow over a portion of the wing behind it. This accelerated airflow increases lift on that side of the aircraft by increasing the pressure differential between the upper and lower wing surfaces.
In normal twin-engine operation, the accelerated slipstream from each propeller increases lift on the corresponding wing, and these rolling effects are balanced. However, when one engine becomes inoperative, this balance is lost, and the effect of accelerated slipstream becomes a significant factor in aircraft controllability.
In a conventional twin-engine airplane, the descending blade of each propeller directs accelerated airflow primarily over the inboard section of the wing behind the operating engine. This localized increase in lift creates a rolling moment away from the operating engine and toward the inoperative engine.
When the left engine fails, the accelerated slipstream from the remaining right engine increases lift on the right wing. This causes the airplane to roll left—toward the inoperative engine. The pilot must counteract this rolling tendency using aileron input, which increases drag and further degrades performance during an engine-out condition.
When the right engine fails, the accelerated slipstream from the left engine produces a rolling moment that is less severe and easier to manage. This difference is one of the reasons why accelerated slipstream contributes to identifying the left engine as the critical engine in a conventional twin.
The effect of accelerated slipstream is strongest at high power settings and low airspeeds, such as during takeoff and initial climb. These are the same conditions under which controllability margins are smallest, making accelerated slipstream a critical consideration in engine-out scenarios.
Understanding how accelerated slipstream influences roll control helps explain why the critical engine creates the most demanding control condition when it fails, particularly during low-speed, high-power phases of flight.
Spiraling Slipstream and the Critical Engine (Yaw)
A spiraling slipstream is the corkscrew-shaped airflow that moves rearward and to the right after leaving the propeller. This spiraling airflow strikes the vertical stabilizer and rudder at an angle, producing a yawing moment on the aircraft.
In a conventional twin-engine airplane, both propellers rotate clockwise, causing the spiraling slipstream from each engine to move toward the right side of the fuselage. Under normal operating conditions, the yawing effects produced by each spiraling slipstream tend to balance each other.
When one engine becomes inoperative, the effect of spiraling slipstream changes and plays a unique role in determining the critical engine. Unlike P-Factor and accelerated slipstream, the spiraling slipstream can partially counteract the airplane’s natural tendency to yaw and roll toward the inoperative engine.
In the case of a left engine failure, the spiraling slipstream from the operating right engine strikes the vertical stabilizer in a manner that produces a yawing force away from the inoperative engine. This yawing moment helps reduce the overall yaw toward the failed engine, slightly improving directional controllability.
If the right engine fails, the remaining left engine’s spiraling slipstream provides less beneficial yaw correction. As a result, the stabilizing effect is reduced, and the aircraft experiences a greater yawing tendency toward the inoperative engine.
Because the spiraling slipstream offers a stabilizing influence during a left engine failure, it reduces the severity of the yawing moment compared to other aerodynamic factors. For this reason, spiraling slipstream does not increase the adverse effects associated with the critical engine and, in fact, partially mitigates them.
Understanding the role of spiraling slipstream is essential for recognizing how different aerodynamic forces interact during an engine-out condition and why some effects intensify the controllability challenge of the critical engine, while others help limit it.
Torque Effect and the Critical Engine (Roll)
The torque effect is a fundamental principle of physics described by Newton’s Third Law, which states that for every action there is an equal and opposite reaction. In an airplane, as the propeller crankshaft rotates clockwise, an opposing force is generated that causes the aircraft to roll counterclockwise.
In a twin-engine airplane with both engines operating, the torque produced by each engine is balanced, and no net rolling tendency exists. However, when one engine becomes inoperative, the remaining engine’s torque is no longer opposed, and its rolling effect becomes significant.
In a conventional twin-engine airplane, the operating engine produces a torque-induced rolling tendency toward the failed engine. When the left engine fails, the right engine’s clockwise propeller rotation causes the aircraft to roll left, further increasing the tendency to roll into the inoperative engine. This rolling moment must be counteracted with aileron input, which increases drag and reduces overall performance.
When the right engine fails, the torque produced by the remaining left engine still causes a left rolling tendency, but the magnitude of this effect is less adverse due to the relative position of the thrust line and the interaction with other aerodynamic factors. As a result, the roll induced by torque is easier to control.
Although torque alone does not determine the critical engine, it contributes to the overall rolling tendency that occurs during an engine-out condition. When combined with P-Factor and accelerated slipstream, torque increases the control challenges associated with the failure of the critical engine.
Understanding torque effect is essential for recognizing how multiple aerodynamic forces act together to influence aircraft controllability and why the loss of the critical engine produces the most demanding roll control scenario in a conventional twin-engine airplane.
Conclusion
Understanding how the critical engine is determined is essential for safe and effective operation of a twin-engine aircraft. The PAST factors—P-Factor, Accelerated Slipstream, Spiraling Slipstream, and Torque—work together to explain why the loss of one engine creates a more demanding controllability challenge than the loss of the other.
In a conventional twin-engine configuration, these aerodynamic effects combine to make the failure of the left engine the most adverse condition from a controllability standpoint. This understanding is not merely theoretical; it directly influences pilot training, certification standards, and real-world engine-out decision-making.
By recognizing how each aerodynamic factor contributes to yaw and roll during asymmetric thrust, pilots gain a clearer understanding of why maintaining directional control becomes most critical during low-speed, high-power phases of flight. Mastery of the critical engine concept allows pilots to anticipate aircraft behavior, apply correct control inputs, and manage engine-out situations with greater confidence and precision.
For a broader operational perspective on twin-engine safety and extended-range considerations, you may also find this article useful:
👉 https://melibrary.pro/article/etops-twin-engine-operations/
